Transistor lights up

Editor's note: this research has been withdrawn by the scientists.

Researchers from Bell Labs have coaxed a transistor to emit light, a feat that could eventually lead to lasers made from organic materials, and alternative printing and optical data storage technologies.

The researchers used a field-effect transistor with an organic material sandwiched among its three electrodes. Organic materials are difficult to make emit light because they do not conduct current well. Scientists have focused on using them for lasers, however, because they would make for cheaper, more flexible lasers than the ceramics used now.

"To the best of our knowledge this is the first field-effect transistor which actually emits light," said Jan Hendrick Schön, a physicist at Bell Labs. The effect could be used to make organic lasers whose light originates from electricity rather than from another light source, said Schön. The device the researchers demonstrated was based on the organic semiconductor -sexithiophene.

Electrons usually enter the field-effect transistor through one electrode and exit through a second one. The third electrode is used as a gate to control the flow of electrons. Electroluminescence, however, happens when electrons meet holes, which have an opposite positive charge.

The researchers added holes to the equation by flowing electrons through the entrance electrode, and at the same time flowing a current of holes through the exit electrode. When the two charges met in the middle they formed excitons, which emit light.

"It's a new way of making LED's... a completely new architecture for an LED, and what it will turn into, we'll see," said Mark Thompson, a chemistry professor at the University of Southern California. Traditional organic LED design has the organic material sandwiched between two electrodes.

The design potentially allows more current to pass through the organic semiconductor, said Thompson. "I'm sure what they're hoping is by using the [field-effect transistor] structure they can get higher conductivity [in organic materials], which means a more efficient device, which means higher brightness and a lower current density,” said Thompson.

This is important because organic materials "are pretty bad conductors to start with -- everything we make organic LEDs out of are insulators," Thompson added.

The design "might also be advantageous for inorganic materials which are difficult to dope," said Schön.

In addition, it could potentially be used as an array of lasers on flexible, large area substrates for printing or optical data storage technologies, he said. An array of organic lasers could be "useful for optical storage, since the number of lasers writing and reading the data could be increased," said Schön.

The next step is to demonstrate an electrically-pumped organic thin film laser based on the light-emitting field-effect transistor, said Schön. In addition, "we are looking into new materials which emit light of different colors," he said.

It will be at least two years before any practical device can be made using the transistor, said Schön. "Questions which have to be addressed include low-cost processing, stability [and] reliability. The progress on organic LEDs suggests that those difficulties might be solved," he said.

Schön's research colleagues were Ananth Dodabalapur, Christian Kloc and Bertram Batlogg. They published a technical paper about the research in the November 3, 2000 issue of Science. The research was funded by Lucent Technologies' Bell Laboratories.

Timeline:  >2 years
Funding:   Corporate
TRN Categories:  Semiconductors and Materials; Integrated Circuits; Optical Computing, Optoelectronics and Photonics
Story Type:   News
Related Elements:  Technical paper, "A Light-Emitting Field-Effect Transistor," Science, November 3, 2000